critique my setup -- weak link? best way to improve?

Your biggest limiting factor is placing your speakers on a sideboard.  The best/cheapest thing you can do by far is get your speakers on some stands and position them properly.  Upgrading speakers would be an obvious area for significant improvement, but if aesthetics are more important to you than sound quality and the speakers need to stay where they are I wouldn’t spend a lot because the sound will always be compromised.  There are some speakers that are designed to work better close to walls, in cabinets, etc. so I’d look to something like that or at least a speaker that’s sealed or front ported.  Best of luck. 

you could also add a sub. I think the big question, are you a "set and forget" type or a fiddler. If you are the former, you can get powered active speakers and sell the Rotel and also upgrade your cartridge. Pro-ject tonearms are good but you can improve the needle. And yes, lastly, a phono stage

your first step should be upgrading your loudspeakers 

atc scm 40s are remarkable and at 7k a pair not expensive 

they can handle any music with ease

Dave and Troy

audio intellect NJ

ATC dealers

You can get rid of the passive preamp. You might like the Schitt Kara.

atc scm 40s are remarkable and at 7k a pair not expensive

are you high? 7K is not expensive? Have you read what he has? His entire system doesn’t add up to 2K, how is it a sensible suggestion to make for seven grand? Are you listing whatever you have in stock randomly or actually caring about OP’s question?